1. Field of the Invention
The present invention relates generally to the field of emission control equipment for boilers, heaters, kilns, or other flue gas-, or combustion gas-, generating devices (e.g., those located at power plants, processing plants, etc.) and, in particular to a new and useful method and apparatus for preventing the plugging, blockage and/or contamination of an SCR catalyst. In another embodiment, the method and apparatus of the present invention is designed to protect an SCR catalyst from plugging and/or blockage from large particle ash that may be generated during combustion.
2. Description of the Related Art
NOx refers to the cumulative emissions of nitric oxide (NO), nitrogen dioxide (NO2) and trace quantities of other nitrogen oxide species generated during combustion. Combustion of any fossil fuel generates some level of NOx due to high temperatures and the availability of oxygen and nitrogen from both the air and fuel. NOx emissions may be controlled using low NOx combustion technology and post-combustion techniques. One such post-combustion technique is selective catalytic reduction using an apparatus generally referred to as a selective catalytic reactor or simply as an SCR.
SCR technology is used worldwide to control NOx emissions from combustion sources. This technology has been used widely in Japan for NOx control from utility boilers since the late 1970's, in Germany since the late 1980's, and in the US since the 1990's. The function of the SCR system is to react NOx with ammonia (NH3) and oxygen to form molecular nitrogen and water. Industrial scale SCRs have been designed to operate principally in the temperature range of 500° F. to 900° F., but most often in the range of 550° F. to 750° F. SCRs are typically designed to meet a specified NOx reduction efficiency at a maximum allowable ammonia slip. Ammonia slip is the concentration, expressed in parts per million by volume, of unreacted ammonia exiting the SCR.
For additional details concerning NOx removal technologies used in the industrial and power generation industries, the reader is referred to Steam: its generation and use, 41st Edition, Kitto and Stultz, Eds., Copyright© 2005, The Babcock & Wilcox Company, Barberton, Ohio, U.S.A., particularly Chapter 34—Nitrogen Oxides Control, the text of which is hereby incorporated by reference as though fully set forth herein.
Regulations (March 2005) issued by the EPA promise to increase the portion of utility boilers equipped with SCRs. SCRs are generally designed for a maximum efficiency of about 90%. This limit is not set by any theoretical limits on the capability of SCRs to achieve higher levels of NOx destruction. Rather, it is a practical limit set to prevent excessive levels of ammonia slip. This problem is explained as follows.
In an SCR, ammonia reacts with NOx according to the following stoichiometric reactions (a) to (c):4NO+4NH3+O2→4N2+6H2O  (a)12NO2+12NH3→12N2+18H2O+3O2  (b)2NO2+4NH3+O2→3N2+6H2O  (c).
The above reactions are catalyzed using a suitable catalyst. Suitable catalysts are discussed in, for example, U.S. Pat. Nos. 5,540,897; 5,567,394; and 5,585,081 to Chu et al., all of which are hereby incorporated by reference as though fully set forth herein. Catalyst formulations generally fall into one of three categories: base metal, zeolite and precious metal.
Base metal catalysts use titanium oxide with small amounts of vanadium, molybdenum, tungsten or a combination of several other active chemical agents. The base metal catalysts are selective and operate in the specified temperature range. The major drawback of the base metal catalyst is its potential to oxidize SO2 to SO3; the degree of oxidation varies based on catalyst chemical formulation. The quantities of SO3 which are formed can react with the ammonia carryover to form various ammonium-sulfate salts.
Zeolite catalysts are aluminosilicate materials which function similarly to base metal catalysts. One potential advantage of zeolite catalysts is their higher operating temperature of about 970° F. (521° C.). These catalysts can also oxidize SO2 to SO3 and must be carefully matched to the flue gas conditions.
Precious metal catalysts are generally manufactured from platinum and rhodium. Precious metal catalysts also require careful consideration of flue gas constituents and operating temperatures. While effective in reducing NOR, these catalysts can also act as oxidizing catalysts, converting CO to CO2 under proper temperature conditions. However, SO2 oxidation to SO3 and high material costs often make precious metal catalysts less attractive.
As is known to those of skill in the art, various SCR catalysts undergo plugging and/or poisoning when they become contaminated by various compounds including, but not limited to, ash from the combustion process (in particular coal ash). One common source of plugging in SCRs is large particle ash (typically defined as any ash that has a particle size large enough to lodge in the catalyst passages, pores, or honeycomb structure present in the SCR catalyst blocks).
Given the above, a need exists for a system and method that can prevent the plugging and/or poisoning of a catalyst in an SCR with fly ash, particularly large particle ash.